1. Field of the Invention
The present invention relates to a detector and charged particle beam instrument.
2. Description of Related Art
Scintillators which emit light when irradiated with an electron beam are used in dark- field detectors and secondary electron detectors which are used respectively in scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM).
For example, JP-A-2013-26152 discloses an electron detector which is fitted with a ceramic scintillator including a ceramic phosphor prepared by sintering P47 (Y2SiO5:Ce) and which is for use in electron microscopy.
A scintillator made of a different material or shaped differently is used in a different detector application. Known examples of such scintillators include single crystal scintillators and powdered scintillators.
For example, where a single crystal scintillator is used and an electron beam is accelerated by an accelerating voltage, as the accelerating voltage is increased, the brightness of the scintillator increases. Therefore, an electron detector using a single crystal scintillator provides high detection sensitivity under high accelerating voltages. However, the detection sensitivity decreases with reducing the accelerating voltage.
Furthermore, a powdered scintillator shows a brightness peak under a certain accelerating voltage although this phenomenon is affected by the thickness of the scintillator, for the following reason. As the accelerating voltage decreases, the electron beam transmits through the scintillator less easily. This in turn deteriorates the brightness. As the accelerating voltage is increased, a greater portion of the beam transmits through the scintillator, leading to a decrease in the brightness. The value of the accelerating voltage at which the peak occurs varies with thickness of the powdered scintillator. An electron detector using a powdered scintillator shows a high detection sensitivity at the accelerating voltage where the peak occurs. The detection sensitivity decreases in going away from the accelerating voltage at which the accelerating voltage shows a peak.
In this way, the above-described electron detector has the problem that its detection sensitivity is not high over a wide range of accelerating voltages from a low voltage to a high voltage.